Measurement of the azimuthal anisotropy of charged particles in s N N = 5.36 TeV O 16 + O 16 and Ne 20 + Ne 20 collisions with the ATLAS detector

UNSPECIFIED (2026) Measurement of the azimuthal anisotropy of charged particles in s N N = 5.36 TeV O 16 + O 16 and Ne 20 + Ne 20 collisions with the ATLAS detector. Physical Review C, 113 (4): 045205. ISSN 2469-9985

Abstract

This paper presents the first measurements of the azimuthal anisotropy coefficients v n , which quantify the n th -order Fourier modulation of charged-particle azimuthal distributions, for n = 2 − 4 in s N N = 5.36 TeV O 16 + O 16 and Ne 20 + Ne 20 collisions recorded with the ATLAS detector at the CERN Large Hadron Collider in 2025. The v n coefficients are measured as a function of transverse momentum ( p T ), collision centrality, and event multiplicity. They are extracted using two complementary methods: two-particle correlations with a template-fit subtraction of short-range nonflow contributions, and four-particle subevent cumulants, which intrinsically suppress nonflow effects and provide sensitivity to flow fluctuations. The results show a clear hierarchy v 2 > v 3 > v 4 and a nonmonotonic dependence on p T , reaching a maximum around 2 GeV , consistent with trends observed in heavy-ion collisions. Detailed comparisons between the two collision systems reveal an enhanced v 2 in central Ne 20 + Ne 20 collisions, consistent with theory expectations based on the predicted prolate deformation of neon nuclei, in contrast to the slightly tetrahedral structure predicted for oxygen. The four-particle cumulant results highlight strong event-by-event fluctuations and provide the greatest sensitivity to nuclear shape effects. These measurements can place new constraints on the initial geometry and the hydrodynamic response in light-ion collisions, offering valuable input for models of nuclear structure.